Rapid thermal processing (RTP) is a term applied to several types of thermal processes including annealing, dopant activation, oxidation, and nitridation among others. The aforementioned processes are typically performed at relatively high temperatures above about 1000° C. It can be further applied to etching and chemical vapor deposition in the presence of precursor or etching gases. The latter processes are conventionally performed in an RTP chamber at somewhat lower temperatures of between 500° and 800° C. RTP typically depends upon an array of high-intensity incandescent lamps fit into a lamphead and directed at the substrate being processed. The lamps are electrically powered and can be very quickly turned on and off and a substantial fraction of their radiation can be directed to the substrate. As a result, the wafer can be very quickly heated without substantially heating the chamber and thereafter can be nearly as quickly cooled once the power is removed from the lamps. Thereby, the processing time at a predetermined temperature can be more closely controlled and the total thermal budget can be reduced. Furthermore, the total processing time can be reduced, thereby increasing throughput.
FIG. 1 schematically illustrates in cross section an RTP reactor 10 described by Ranish et al. in U.S. Pat. No. 6,376,804, incorporated herein by reference and generally representative of the Radiance RTP reactor available from Applied Materials, Inc. of Santa Clara, Calif. The reactor 10 includes a processing chamber 12, a wafer support 14 located within the chamber 12, and a lamphead 16 or heat source assembly located on the top of the chamber 12, all generally symmetrically arranged about a central axis 18.
The processing chamber includes a main body 20 and a window 22 resting on the main body 20. The window 22 is made of a material that is transparent to infrared light, for example, clear fused silica quartz.
The main body 20 is made of stainless steel and may be lined with quartz (not shown). An annular channel 24 is formed near the bottom of the main body 20. The wafer support 14 includes a rotatable magnetic rotor 26 located within the channel 24. A quartz tubular riser 28 rests on or is otherwise coupled to the magnetic rotor 26 and an edge ring 30 composed of silicon-coated silicon carbide, opaque silicon carbide or graphite rests on the riser 28. During processing, a wafer 32 or other substrate rests on the edge ring 30. A rotatable magnetic stator 34 is located externally of main body 20 in a position axially aligned with the magnetic rotor 24 and is magnetically coupled to it through the main body 18. An unillustrated motor rotates the magnetic stator 34 about the central axis 18 and thereby rotates the magnetically coupled rotor 26 and hence rotates the edge ring 28 and the supported wafer 30. Three or four lift pins 36 are slidably sealed to a reflector plate 38 forming a bottom wall of the main body 20. An unillustrated mechanism lifts and lowers all the lift pins 36 to selectively engage the wafer 22 to lower and raise it to and from the edge ring 30 and to and from a unillustrated paddle used to transfer of the wafer 22 into and out of the chamber 12.
The quartz window 22 rests on an upper edge of the main body 18 and an O-ring 40 located between the window 22 and the main body 20 provides an air-tight seal between them. The lamphead 16 overlies the window 22. Another O-ring 42 located between the window 22 and the lamphead 16 provides an airtight seal between them. Clamps 44 in conjunction with the O-rings 40, 42 thus seals the lamphead 16 to the main body 20.
The lamphead 16 includes a plurality of lamps 46 that are supported by and electrically powered through electrical sockets 48. The lamps 46 are preferably high-intensity incandescent lamps that emit strongly in the infrared such as tungsten halogen bulb having a tungsten filament inside a quartz bulb filled with a gas containing a halogen gas such as bromine and diluted with an inert gas to clean the quartz bulb. Each bulb is potted with a ceramic potting compound 50, which is relatively porous. The lamps 46 are located inside vertically oriented cylindrical lamp holes 52 formed in a reflector body 54. The open ends of the lamp holes 52 of the reflector body 54 are located adjacent to the window 22 with the lamps 46 separated from the window 22.
A liquid cooling channel 56 is formed within the reflector body 54 to surround each of the lamp holes 52. A coolant, such as water, introduced into the cooling channel 56 via an inlet 60 and removed at an outlet 62, cools the reflector body 54 and flowing adjacent the lamp holes 52 cools the lamps 46.
Thermal sensors such as seven or more pyrometers 70 are optically coupled by light pipes 72, such as sapphire rods, to respective apertures 72, which are formed through and are disposed and spaced across the radius of the reflector plate 38. Typically the rigid sapphire light pipes 72 and pyrometers are supported in the main body 20 but there may be an intermediate flexible optical fiber or light guide. The pyrometers 70 detect a temperature or other thermal property of a different radial portion of the lower surface of the wafer 30 and edge ring 30, as Peuse et al. describe in U.S. Pat. No. 5,755,511. Adams et al. describe such a pyrometer in U.S. Pat. No. 6,406,179, incorporated herein by reference in its entirety. The pyrometer 70 is more particularly a radiation pyrometer and includes an optical narrow-band filter having a passband of about 20 nm at a wavelength less than 950 nm, that is, at a photon energy somewhat above the silicon band gap of about 1.1 eV (1.1 μm), alternately expressed as photon wavelength below the band gap wavelength of the silicon wafer. Such filters are easily formed as multi-layer interference filters. Thereby, the silicon wafer 32 absorbs the shorter-wavelength visible radiation emitted from the lamps 46 so that the pyrometer 70 is sensitive to the blackbody radiation emitted from the wafer 32 rather than the radiation from the lamps 46.
The pyrometers 72 supply temperature signals to a lamp power supply controller 76, which controls the power supplied to the infrared lamps 46 in response to the measure temperatures. The infrared lamps 46 may be controlled in radially arranged zones, for example, fifteen zones, to provide a more tailored radial thermal profile to account for thermal edge effects. The pyrometers 72 together provide signals indicative of a temperature profile across the wafer 22 to the power supply controller 76, which controls the power supplied to each of the zones of the infrared lamps 46 in response to the measured temperatures, thus providing a closed loop thermal control.
The main body 20 of the processing chamber 12 includes a processing gas inlet port 80 and a gas outlet port 82. In use, the pressure within the processing chamber 12 can be reduced to a sub-atmospheric pressure prior to introducing a process gas through the inlet port 80. A vacuum pump 84 evacuates the process chamber 86 by pumping through a port 76 and a valve 88. The pressure is typically reduced to between about 1 and 160 torr. Certain processes, however, can be run at atmospheric pressure, though often in the presence of a specified gas, and the process chamber does not need to be evacuated for such processes.
A second vacuum pump 90 reduces the pressure within the lamphead 16, particularly when the processing chamber is pumped to a reduced pressure to reduce the pressure differential across the quartz window 22. The second vacuum pump 90 reduces the pressure within the lamphead 16 by pumping though a port 92 including a valve 94. The port 92 is in fluid communication with an interior space of the reflector body 54 including the lamp holes 52.
A pressurized source 98 of a thermally conductive gas, such as helium, fills the lamphead 16 with the thermally conductive gas to facilitate thermal transfer between the lamps 46 and the liquid cooling channels 56. The helium source 98 is connected to the lamphead 16 through a valve 100 and port 102. The thermally conductive gas is introduced into a gas manifold 104 formed between a lamphead cover 106 and the base of each lamp 46. Opening the valve 100 causes the gas to flow into the manifold 104. Since the lamp potting compound 50 is relatively porous, the thermally conductive gas flows through the potting compound 50 and in the gap between the walls of the lamp 46 and the lamp hole 52 to cool the lamps 46.
The described RTP chamber, however, suffers some drawbacks in its use at lower temperatures. The typical radiation pyrometer used for silicon RTP includes a silicon photodiode detector, which detects the intensity of usually a narrow bandwidth of the Plankian radiation spectrum emitted from a hot body and determines the temperature of that body from the detected intensity. However, pyrometry is generally used for measuring high temperatures, for example, above 500 or 800° C. In the configuration of RTP reactors in which the chamber parts are relatively warm and there is light leakage from radiant bulb, conventional pyrometry is relatively ineffective at wafer temperatures of less than about 450° C. Photocurrents of a conventional pyrometer exposed to a 350° C. body are in the neighborhood of 0.8 pA, a level easily overwhelmed by thermal and electronic noise in a typical RTP environment. Furthermore, the wafer is partially transparent at these temperatures and the chamber is not light tight. It has been observed that immediately after the incandescent lamps are turned on in the presence of a cold wafer, the pyrometers immediately register about 350° C. from the direct and indirect lamp radiation.
Low-temperature control of wafer temperatures occurs in at least two situations for RTP. In high-temperature RTP, the higher wafer temperatures are very finely controlled by a closed loop control system using the radiation pyrometers, which, as explained above, are effective only above about 450° C. To reach this temperature, however, the wafer must first be heated under an open loop control system in which predetermined amounts of current are supplied to the radiant lamps. When the pyrometers detect that the temperature has reached a lower detection limit for the radiant pyrometers, thermal control switches to the closed loop system. The pre-heating during the open-loop period is not closely monitored beyond typically a switch off condition. As a result, temperature gradients or excessive heating rates may occur. The wafer may become misshapen during the pre-heating into a domed or potato-chip shape that prevents effective RTP at yet higher temperatures. It has thus been necessary to closely optimize the pre-heating conditions, particularly the distribution of zonal heating to achieve uniform pre-heating. Such pre-heating optimization has conventionally required a skilled engineer to experiment with a large number of wafer to establish a pre-heating recipe which avoids warpage or other deleterious results. However, the optimized recipe strongly depends on the features already established on the wafer. Except in the situation of very long production runs, it is infeasible to optimize for each level of each chip design. Instead, the optimization is performed on a few classes of unpatterned stock wafers having a top layer of a given type of material, for example, either metal or oxide. For production, the pre-heating recipe is selected for a similar top layer. Generally, this approach has proven unsatisfactory and results in uncertain pre-heating rates and other uniformities requiring yet further adjustments.
A demand has arisen for RTP performed at temperatures below 500° C. and even below 250° C. to nearly room temperature, for example, in nickel, cobalt, or titanium silicide contacts being envisioned for future generations of integrated circuits. It would be convenient to apply conventional radiation pyrometry to these advanced processes requiring relatively low thermal treatment temperatures. It is conceivable to design an automated low-temperature RTP chamber with cold walls and low-temperature radiation pyrometers, but it is more desirable to adapt commercialized high-temperature RTP chambers for low-temperature processing. It is further desired to provide an RTP chamber that can be used for both low-temperature and high-temperature processing so that the same chamber can be used for different processing steps.
Hunter et al. have described in U.S. Pat. No. 6,151,446, incorporated herein by reference in its entirety, a transmission pyrometer useful for determining when a wafer supported on lift pins induces enough photocurrent in a photodetector to generally indicate that the wafer has achieved a chamber temperature before the wafer is lowered onto the edge ring. The transmission pyrometer includes some sort of filtering effective within a band near the silicon band gap. As the silicon wafer warms up, its band gap energy decreases (wavelength increases). The transmission pyrometer is intended to detect the radiation from the radiant heating lamps, usually held at a low intensity, as filtered by the silicon wafer. As the silicon band gap passes into or out of the detector's bandwidth, the detector signal significantly changes, thereby providing an indication of the temperature of the silicon wafer. The Hunter transmission pyrometer was incorporated into the chamber's lift pins to determine when it is safe to lower the wafer onto the warm edge ring. It is described as operating only up to about 400° C. Although Hunter provides some feedback control of the lamp power, closer and finer control of wafer temperature is desired.